1. Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same. More particularly, the present invention relates to a negative active material for a rechargeable lithium battery that can improve stability at the interface between a negative electrode and an electrolyte, charge and discharge efficiency, and cycle-life, and can be applied along with many kinds of binders, a method of preparing the same, and a rechargeable lithium battery including the same.
2. Description of the Related Art
A lithium rechargeable battery has recently drawn attention as a power source for a small portable electronic device. It uses an organic electrolyte solution and thereby has a discharge voltage that is twice as high as a conventional battery that uses an alkali aqueous solution, and accordingly has a high energy density.
A negative active material for a lithium battery can reversibly receive and supply lithium ions while maintaining its structural and electrical properties. For the negative active material, a lithium metal, a lithium-containing alloy, or a carbon-based material, which has a chemical potential that is a similar level to lithium metal when lithium ions are intercalated/deintercalated, is usually used. A battery using lithium metal or a lithium alloy as a negative active material is called a lithium metal battery, and a battery using a carbon material is called a lithium ion battery.
A lithium metal battery using lithium metal or a lithium alloy for its negative electrode has a risk of explosion due to a battery short that may be caused by formation of dendrites. Hence, the lithium metal battery is being replaced by a lithium ion battery that uses a carbon material as its negative active material and does not have such a risk. The lithium ion battery only transfers lithium ions during charge/discharge, and it maintains the electrode active material as it is. Thus, the lithium ion battery has improved stability and cycle-life.
In the lithium ion battery, a non-aqueous system in which polyvinylidene fluoride (PVDF) is dissolved in N-methyl-2-pyrrolidone (NMP) or an organic acetone solvent is usually used. The non-aqueous system is widely known as a binder for a conventional negative electrode plate. However, when the PVDF/NMP non-aqueous system is used as a binder, the organic solvent such as NMP and acetone may contaminate the natural environment. Also, since the organic solvent is expensive, the manufacturing cost for a lithium battery is increased. In addition, since most organic solvents are highly volatile, there is a risk of explosion when they are used in an enclosed space. To prevent explosions, an anti-explosion unit is additionally required.
To overcome the problems, researchers have actively studied to develop a method of using an aqueous system in which styrene-butadiene rubber (SBR) is dispersed in water along with a thickener, e.g., carboxylmethylcellulose (CMC), when a negative electrode plate is manufactured. Since the SBR binder can be dispersed in water in the form of emulsion, it does not have to use an organic solvent, and the SBR binder has strong adherence. With the SBR binder, the amount of a binder is reduced and the amount of a negative active material is increased, which is advantageous in manufacturing a high-capacity lithium battery.
When an aqueous solvent is used with an aqueous binder, the high surface tension of the aqueous solvent degrades the coating property of negative active material slurry to thereby reduce the planarity of a negative electrode. The low planarity of the negative electrode makes the surface of the negative electrode non-uniform to thereby reduce a reaction surface area and deteriorate the cycle-life characteristic.